Setting Code 78 and Heavy Black Smoke

Written by Bill Heath

Volume 2 Issue 3 - 6.2/6.5 Issues and Answers

I have been having a problem with my 1995 GMC 3500 HD dump truck and maybe you will be able to help me out. For some time now, my truck has been having a service engine light. I had it checked and find that it is setting a code # 78. When it does this, it also smokes heavy, black smoke. Our Chevy dealer checked it out and replaced the boost sensor on the top of the engine. However, this repair did not fix the problem and he tells me he is stumped. A mechanic friend suggested that it was a bad vacuum valve and that we ought to tie the boost lever with wire to hold it closed. The truck no longer smokes, but it still turns on the service engine light and sets code #78. So now, I don’t know what to think about this problem. Is it too much boost or too little? We seem to be out of options. Can you help?

Based on your description of the problem, it appears that the